The invention relates to optical glass, in particular optical glass for fabricating optical fiber. More especially, the invention relates to Ga:La:S (GLS) glass and related compounds, and to optical fiber and optical devices using such glass.
The production of Ga:La:S (GLS) based fiber without devitrification is an issue that needs addressing. Reaching fiber-drawing viscosities between 104–106 poise at temperatures much less than the onset of crystallization has proven difficult. The high melting temperature (1150° C.) required for the processing of Ga:La:S glass restricts the choices of additives that can be used to act as modifiers.
Advantages of Ga:La:S based glass over other competing glass for active and infrared applications are evident through its low-phonon energy, high rare-earth solubility, high glass transition temperature and non-toxicity. However, the tendency to devitrify during fiber drawing hinders the production of small core fiber. Improving thermal properties of Ga:La:S glasses through addition of selected compounds may hold the key to achieving practical fiber.
The addition of a modifier into the Ga:La:S glass matrix provides a way to achieve the improvements required. Modifiers can introduce higher thermal stability for potential fiber drawing and the ability to transmit further into the visible spectrum. One important goal is to provide a GLS glass capable of shifting the Pr3+ emission peak at 1.3 μm closer to the all important 2nd telecommunications window.
For active applications such as fiber amplifiers, these compositional modifications are also beneficial through the reduction of oxide in the glass. Most importantly, it is important that modified Ga:La:S glasses still retain the key characteristics of Ga:La:S.
Some Ga:La:S modifiers that have been studied are as follows [4]:
GLS modifiersModifierChange to GlassLanthanum oxideImproved thermal stability, visible transmissionCerium sulfideImproved thermal stabilityAluminum sulfideImproved thermal stability, visible transmissionIndium sulfideImproved thermal stabilityLanthanum fluorideImproved thermal stability, visible transmission,purificationBismuth sulfideRaises refractive indexGermaniumNo obvious improvementSodiumImproved low loss fiber
Further work by J. Wang et. al., [11] has studied the effect of adding Cesium Chloride (CsCl) as a modifier to GLS. GLS:CsCl glasses were characterized to show low-phonon energy and high rare-earth solubility while providing improved thermal & optical properties over GLS. Furthermore, GLS:CsCl glasses exhibited blue shifting into the visible, advantageous for active applications. GLS was successfully doped with up to 30 mol % of CsCl. However, 25 mol % was found to be the optimum as the fiber-drawing capabilities were improved considerably. Initial fiber attenuation measurements revealed losses of 10 dB/m at 1.3 μm. In fluorescence measurements for the Pr3+ ion, both GLS and GLS:CsCl had peak emission at 1.34 μm. A serious drawback with GLS:CsCl glasses during bulk production (of about 170 g), is the shattering of glass ingots in the carbon boats due to its high expansion coefficient.
Other work by J Wang et al [12] studied the effects of adding various lanthanum compounds to GLS, namely LaF3, LaCl3, LaBr3 and LaI3. The effect of adding the halides was characterized in terms of the stability parameter Tx-Tg obtained from differential thermal analysis (DTA) studies. The authors reported that the addition of increasing amounts of LaF3 causes a constant deteriorating effect in the thermal stability of the glasses, whereas the addition of LaCl3 or LaBr3 causes an improving effect on the thermal stability up to approximately 8 mol %, with a peak at around 2 to 3 mol %. The addition of LaI3 into the GLS initially causes a deteriorating effect on thermal stability up to 8 mol %, then it starts to show an improving effect on thermal stability with a peak around 20 mol %. In summary, this work showed that addition of large amounts of LaI3 or small amounts of LaCl3 or LaBr3 may be beneficial to GLS, whereas addition of LaF3 to GLS is harmful.
There is still a need for discovering new GLS compositions that provide some improved properties, while retaining the key characteristics of basic Ga:La:S glass.